Collective excitations in electron-hole bilayers

TL;DR

This paper combines analytic and molecular dynamics methods to analyze collective excitations in strongly coupled electron-hole bilayers, revealing an energy gap, acoustic modes, and nonlinear harmonic generation, with comparisons to solid-phase phonons.

Contribution

It provides a novel combined analytic and simulation study of collective modes in electron-hole bilayers, highlighting the energy gap and nonlinear effects in the strong coupling regime.

Findings

Identification of an isotropic energy gap in out-of-phase spectra

Observation of acoustic dispersion in in-phase spectra

Detection of strong nonlinear harmonic generation

Abstract

We report a combined analytic and Molecular Dynamics analysis of the collective mode spectrum of an electron-hole (bipolar) bilayer in the strong coupling quasi-classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles; the in-phase spectra exhibit a correlation governed acoustic dispersion for the longitudinal and transverse modes. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed. The mode dispersions in the liquid state are compared with the phonon spectrum in the crystalline solid phase, reinforcing a coherent physical picture.